# Can I cut a mirror with a laser? **URL:** \ **Category:** Factual Questions\ **Created:** [May 18, 2013, 4:28am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657 "2013-05-18T04:28:21Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [May 18, 2013, 4:28am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/1 "2013-05-18T04:28:21Z")
Yes, it’s reflecting the beam, but if I painted it in black… Is all laser-power because of heat - what if I gryo-froze the object, could I cut it at all? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 18, 2013, 4:35am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/2 "2013-05-18T04:35:30Z")
A CO2 laser will cut a mirror very nicely, because glass is a strong absorber at 10.2µ… Mirrors that are designed to reflect laser beams need to be “first surface” - no glass in the way of the beam. If the laser is high enough power, the mirrors need to be able to handle the energy of the beam, or they will become damaged. As for freezing - a cutting laser puts enormous energy into a very small area. There’s no way that simply cooling the object is going to overcome that amount of heating (the temperature at the focus point can be tens of thousands of degrees - a few hundred degrees below room temperature isn’t going to make much difference). ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [May 18, 2013, 6:01am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/3 "2013-05-18T06:01:04Z")
If you paint a mirror black, it is not a mirror any more. Yes, any cutting a laser does is because of the heat it generates, so cooling the mirror will not help you cut it (though, going by what **beowulff** is saying, it won’t hurt much either). ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [May 18, 2013, 6:57am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/4 "2013-05-18T06:57:11Z")
There’s no such thing as a perfect mirror; even the best real world mirrors absorb some of the energy that hits them. So with enough energy you should be able to cut a mirror even with a light frequency it reflects. > [@njtt](#): > > Yes, any cutting a laser does is because of the heat it generates, \ Interestingly enough that’s not actually true, there are lasers that cut by other methods. [Excimer lasers](http://en.wikipedia.org/wiki/Excimer_laser#Major_applications) slice human tissue not by burning the tissue they strike, but by ablation. > [@](#): > > The ultraviolet light from an excimer laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. Thus excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 18, 2013, 2:19pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/5 "2013-05-18T14:19:03Z")
In fact, this is one of the difficulties in working with high-powered lasers: You’ve got to make very low-absorbing optics to make your beam do whatever it is you want, lest they be damaged by it. ---
**Author:** ![Rhythmdvl](https://avatars.discourse-cdn.com/v4/letter/r/85f322/32.png) [@Rhythmdvl](https://boards.straightdope.com/u/Rhythmdvl)\ **Post date:** [May 18, 2013, 4:35pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/6 "2013-05-18T16:35:31Z")
I see a red mirror and I want it painted black… ---
**Author:** ![Fubaya](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@Fubaya](https://boards.straightdope.com/u/Fubaya)\ **Post date:** [May 18, 2013, 5:05pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/7 "2013-05-18T17:05:15Z")
> [@komolono](#): > > Yes, it’s reflecting the beam, but if I painted it in black… > Is all laser-power because of heat - what if I gryo-froze the object, could I cut it at all? Just flip it over and cut from the back 🙂 ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [May 18, 2013, 5:38pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/8 "2013-05-18T17:38:03Z")
> [@Der\_Trihs](#): > > There’s no such thing as a perfect mirror; even the best real world mirrors absorb some of the energy that hits them. So with enough energy you should be able to cut a mirror even with a light frequency it reflects. I have this Gary Larsonesqe vision of a guy intently cutting a mirror with a laser, while pieces fall off of everything else in the room, which is in flames. ---
**Author:** ![drewtwo99](https://avatars.discourse-cdn.com/v4/letter/d/d07c76/32.png) [@drewtwo99](https://boards.straightdope.com/u/drewtwo99)\ **Post date:** [May 18, 2013, 6:16pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/9 "2013-05-18T18:16:18Z")
> [@TimeWinder](#): > > I have this Gary Larsonesqe vision of a guy intently cutting a mirror with a laser, while pieces fall off of everything else in the room, which is in flames. I also picture some other guy just coming over with a glass cutter and cutting another mirror (or the same one the laser is working on perhaps?) in but a moment’s time. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [May 19, 2013, 5:07pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/10 "2013-05-19T17:07:51Z")
> [@Fubaya](#): > > Just flip it over and cut from the back 🙂 The point here is that when you cut the paint, you see the reflecting mirror - so a thin layer of paint might not be enough… ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [May 19, 2013, 11:03pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/11 "2013-05-19T23:03:57Z")
> [@Der\_Trihs](#): > > There’s no such thing as a perfect mirror; even the best real world mirrors absorb some of the energy that hits them. So with enough energy you should be able to cut a mirror even with a light frequency it reflects. > > . This. I was working with a high-power CO2 laser that we built, and was extremely frustrated because it had such a high internal energy density that the beam would seize upon any slight defect and get absorbed by it, thereby degrading it. Thenext shot would degrade it still further, and the subsequent shot would degrade it yet again, and on and on in a vicious cycle that lef me with a ruined mirror after several shots. fter which, of course, the laser didn’t work any more. Extremely frustrating, because it was obvious this would happen. But I didn’t design the laser – I just had to make it work. Often you don’t even need a defect – there are damage thresholds when the mirror itself fails. I’ve had that happen, too. as **Der Trihs** properly says, there’s no such thing as a perfect mirror. And if there was (like one of Larry Niven’s Stasis Fields), you’d find yourselgf running into paradox territory. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [May 19, 2013, 11:16pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/12 "2013-05-19T23:16:54Z")
> [@TimeWinder](#): > > I have this Gary Larsonesqe vision of a guy intently cutting a mirror with a laser, while pieces fall off of everything else in the room, which is in flames. 😃 _\_ ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 20, 2013, 2:49am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/13 "2013-05-20T02:49:18Z")
Nitpick: I don’t think even Niven’s stasis fields were perfect mirrors. I’m pretty sure they have an absorption coefficient equal to their time dilation factor (which is really really close to zero, but not quite equal to it). ---
**Author:** ![Glazer](https://avatars.discourse-cdn.com/v4/letter/g/a88e57/32.png) [@Glazer](https://boards.straightdope.com/u/Glazer)\ **Post date:** [May 20, 2013, 3:32am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/14 "2013-05-20T03:32:06Z")
> [@Chronos](#): > > Nitpick: I don’t think even Niven’s stasis fields were perfect mirrors. I’m pretty sure they have an absorption coefficient equal to their time dilation factor (which is really really close to zero, but not quite equal to it). Now that’s some world class geekery there. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [May 22, 2013, 5:05am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/15 "2013-05-22T05:05:54Z")
What happens when you lase water? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [May 22, 2013, 6:07am UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/16 "2013-05-22T06:07:57Z")
> [@komolono](#): > > What happens when you lase water? You’ve never had Laser Water?! Hot damn, tastes like cinnamon. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [May 22, 2013, 12:57pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/17 "2013-05-22T12:57:07Z")
> [@Chronos](#): > > Nitpick: I don’t think even Niven’s stasis fields were perfect mirrors. I’m pretty sure they have an absorption coefficient equal to their time dilation factor (which is really really close to zero, but not quite equal to it). I’ve read discussions that claimed they were perfectly reflecting. Whence comes your variation? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [May 22, 2013, 12:59pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/18 "2013-05-22T12:59:08Z")
> [@komolono](#): > > What happens when you lase water? You get a Water Vapor Laser: [http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1075731&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs\_all.jsp%3Farnumber%3D1075731](http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1075731&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D1075731) > [@](#): > > Water-vapor lasers oscillate on more than a hundred pulsed lines and over a dozen CW lines over the wavelength range from 7 to 220 μ (1300-45 cm-1). Nearly eighty of these lines are now identified as vibrational-rotational and pure rotational transition in the low-lying vibrational states of the water-vapor molecule. The structure of the water-vapor molecule is reviewed, and the mechanisms responsible for creating population inversions in water-vapor discharges are discussed. It is shown how a model, based on perturbations between rotational levels of different vibrational states, can be used to explain the laser action and to predict new laser lines. Detailed summaries of data on all observed laser lines in H2O16, H2O18, and D2O16and their identifications are presented. Some of the lines are newly reported, and more accurate wavelength measurements have been made for many of the lines. The observation of predicted laser lines in H2O18is shown to be a strong confirmation of the perturbation model. The list of references contains a fairly complete bibliography of previous work on the water-vapor laser, and a representative bibliography of work on the energy-level structure of the water-vapor molecule. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 22, 2013, 3:26pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/19 "2013-05-22T15:26:18Z")
> [@](#): > > I’ve read discussions that claimed they were perfectly reflecting. Whence comes your variation? It might just have been something I came up with, while trying to justify why they’d be (near)-perfect mirrors to begin with. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 22, 2013, 4:29pm UTC](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657/20 "2013-05-22T16:29:17Z")
> [@Chronos](#): > > It might just have been something I came up with, while trying to justify why they’d be (near)-perfect mirrors to begin with. Not to hijack this thread too much, but (as I recall) Niven changed the way his Stasis fields worked over the years. In some of his stories, time completely stopped inside of a field, in others time simply progressed very, very slowly. [Next page](https://boards.straightdope.com/t/can-i-cut-a-mirror-with-a-laser/658657.md?page=2)